help me settle a debate @ my mechanic's shop, sort of a technical question Options

[SirAndy]

I think you mean a piston in a car... (IMG:style_emoticons/default/biggrin.gif)

Uhm, yeah. I thought we were talking motors. (IMG:style_emoticons/default/confused24.gif)

Your vacuum pump isn't open on the other end to let air in. That's a big difference, me thinks.

A piston in an engine does not create a vacuum when moving down. (IMG:style_emoticons/default/poke.gif)

[Jasfsmith]

I think you mean a piston in a car... (IMG:style_emoticons/default/biggrin.gif)

Uhm, yeah. I thought we were talking motors. (IMG:style_emoticons/default/confused24.gif)

Your vacuum pump isn't open on the other end to let air in. That's a big difference, me thinks.

A piston in an engine does not create a vacuum when moving down. (IMG:style_emoticons/default/poke.gif)

Dumb question then, with a vacuum gauge hooked up, what is it measuring?

[RJMII]

Dumb question then, with a vacuum gauge hooked up, what is it measuring?

The difference between ambient pressure and cylinder pressure in mm of mercury?

[Jasfsmith]

Dumb question then, with a vacuum gauge hooked up, what is it measuring?

The difference between ambient pressure and cylinder pressure in mm of mercury?

Then the piston is creating a vacuum. Yes?

[RJMII]

it's creating a lower pressure zone. (IMG:style_emoticons/default/biggrin.gif) for the ambient air to blow into.

[zonedoubt]

As the piston moves away from the valves, volume increases which leads to a pressure drop. Ambient air at a higher pressure moves into the cylinder.

[SirAndy]

Then the piston is creating a vacuum. Yes?

No. A vacuum by definition is void of (almost) any matter.
http://en.wikipedia.org/wiki/Vacuum


The piston in your motor sucks in ambient air, thus your cylinders are not void of matter on the downstroke, they always have air & gasoline in them.
The whole purpose of your motor is to pump air, not to create a vacuum. It's not a vacuum pump, it's a air pump.
(IMG:style_emoticons/default/shades.gif)

[ME733]

............So to summerize ..The engine is an air pump....and when the Exhaust valve and Intake valve are both open, at T.D.C. overlap position.....the exhaust gas scavenging cycle (lets call it)....CAN create a Vaccum in the combustion chamber. This is because the exhaust gases blowing down the exhaust pipes (from the HIGH pressures of ignition, and combustion),Can create a (small) Vaccum. This vaccum can be enhanced by an expertly designed exhaust system, and exhaust port work,(head work) valve guide shaping, valve seat angles,piston dome shape, etc.etc. ..and a proper camshaft precisely timed. The ENGINE in operation can/ will have a negative pressure in the combustion chamber at ..X...engine RPM,s based on the camshaft and many other factors, some mentioned above.

[SirAndy]

I think the confusion comes from the fact that a lot of people (incorrectly) substitute negative pressure with vacuum.

As long as you are moving matter, you don't have a vacuum. A vacuum is the absence of any matter.


The moving pistons and the expanding hot exhaust gases create negative pressure which aids in pumping large amounts of air through your engine.

At no point in the process are you creating any significant amount of empty space that is void of all matter.

Hence, no vacuum. (IMG:style_emoticons/default/biggrin.gif)

[ME733]

.........YES....my post revised....I have substituted vaccum for negative pressure, in the past myself.

[race914]

Check your physics books (or google some info now-a-days)...

There is no such force as 'suction'

Go look up why a well pump can only pull up water equal to the atmospheric pressure....

[SLITS]

Fark Fark .... now I have to take apart all the vacuum gauges I have and remark them "negative pressure" gauges. I wonder if there is enough room on the gauge face to be politically correct.


Gad, I think I will go (IMG:style_emoticons/default/barf.gif)

If the cylinder doesn't produce suction on the downstroke, why do I feel suction on my finger when I stick it in the spark plug hole? Is there a pair of lips in there? Is there a more politically correct single word?

These and all other questions of this nature will be answered in tomorrow's episode .... "Why We Can't Suck; Only Create Negative Pressure"

Oh, and a bumble bee cannot fly!

[SirAndy]

Gad, I think I will go (IMG:style_emoticons/default/barf.gif)

Do you have a "Negative Pressure Cleaner" at your house? (IMG:style_emoticons/default/av-943.gif)

[SLITS]

Gad, I think I will go (IMG:style_emoticons/default/barf.gif)

Do you have a "Negative Pressure Cleaner" at your house? (IMG:style_emoticons/default/av-943.gif)

No, I have a mechanical device that moves the dirt and garbage from the viewing area to underneath the rug. From there, by some mystical & unknown force, it moves to the moon, where it can be found in great piles. On a dark night, if you look carefully with a telescope, you will see the piles.

[Lennies914]

So yellow cars arn't really faster? (IMG:style_emoticons/default/headbang.gif)

[orange914]

it's creating a lower pressure zone. (IMG:style_emoticons/default/biggrin.gif) for the ambient air to blow into.

i remember my shop teacher flogging the term "negative pressure". hows that for a P.C. compromise?

[Tom_T]

I believe that your shop teacher is right. Also explains why a turbo (which you can't use on a 914) will fill more charge into the cylinder as it's under more pressure. The air is moving from high pressure areas to low pressure areas in an attempt to equalize pressures.

The above is assuming that I remember my physics from 40 years ago!

I had heard a rumor somewhere that you can't turbocharge a 914 (I don't remember where) so, thanks for confirming that!

I think a portion of my shop teachers point might have been intake charge variables regarding altitude above sea level and their effect on fuel ratios.

There have been a couple of turbo'd 914's I've seen on here, so search the topics on that & read on, but you'd have to resolve the higher compression ratio issues with a turbo to make it all run reliable I would think.

[Tom_T]

If the cylinder doesn't produce suction on the downstroke, why do I feel suction on my finger when I stick it in the spark plug hole? Is there a pair of lips in there?

Careful Ron, or you'll have some guys sticking all sorts of thangs in there! (IMG:style_emoticons/default/sheeplove.gif)
(IMG:style_emoticons/default/lol-2.gif)

Nice to meet you & chat at GPR Saturday!
.... your string of various past/present "professional endeavors" reminds me of that limerick from the old Dick Van Dyke Show that only us old fartz will remember! (IMG:style_emoticons/default/laugh.gif)

Cheers! (IMG:style_emoticons/default/beerchug.gif)
Tom T
///////

[KaptKaos]

Wait....

Wait....

Wait....

WAIT!!!

Ok, I am a little confused here. I keep reading that there is a vacuum of sorts created by the piston on the down stroke. That this area of low pressure, causes or induces the intake charge to help fill that volume that was evacuated by the piston, right?
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My pistons move sideways, so how does it work then? (IMG:style_emoticons/default/laugh.gif)

[SLITS]

Wait....

Wait....

Wait....

WAIT!!!

Ok, I am a little confused here. I keep reading that there is a vacuum of sorts created by the piston on the down stroke. That this area of low pressure, causes or induces the intake charge to help fill that volume that was evacuated by the piston, right?
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My pistons move sideways, so how does it work then? (IMG:style_emoticons/default/laugh.gif)

You must have Ornamental pistons .... in their part of the world, most things are sideways.